A term possessing genetic implications, originally defined as an aquatic sediment rich in organic matter that formed under reducing conditions (lack of dissolved oxygen in the water column) in a stagnant water body. This contrasts with the term gyttja, which is also a sediment high in organic carbon content but which formed under inferred oxygenated conditions in the water column down to the sediment-water interface (thus benthic organisms may be present).

Such inferences about water-column dissolved-oxygen contents are not always easy to make for ancient environments. Therefore, the term sapropel or sapropelic mud has been used loosely to describe any discrete black or dark-colored sedimentary layers (>1 cm or 0.4 in. thick) that contain greater than 2 wt % organic carbon. Sapropels may be finely laminated (varved) or homogeneous, and may less commonly exhibit structures indicating reworking or deposition of the sediment by currents. Sapropels largely contain amorphous (sapropelic) organic matter derived from planktonic organisms (such as planktonic or benthic algae in lakes or plankton in marine settings). Such organic matter possesses a large hydrogen-to-carbon ratio; therefore, sapropelic sequences are potential petroleum-forming deposits. The enhanced preservation of amorphous organic matter in sapropels may indicate conditions of exceptionally great surface-water productivity, extremely low bottom-water dissolved-oxygen contents, or both. Some sapropels may, however, contain substantial amounts of organic matter derived from land plants.